1. Field of the Invention
The present invention relates to transmissions of the type wherein an input shaft is connected to an output shaft by an endless band received over adjustable pulleys on said shafts. More specifically, the present invention relates to adjustment of tension applied to the endless band.
2. Description of the Prior Art
A well-known form of transmission has parallel input and output shafts on each of which is mounted, in nonrotating relation, a pulley with conical sidewalls shiftable toward and from each other. An endless band, which may be a belt or chain, is received over the adjustable pulleys to transmit torque from the input shaft to the output shaft. In order to change the speed ratio between the input shaft and the output shaft, the effective diameters of both pulleys is changed, one increasing in diameter and the other simultaneously decreasing in diameter, since the endless band is of fixed length. This is usually done by means of two levers, one engaged with one side of each of the pulleys and one engaged with the other side of the pulleys. The levers pivot about a fulcrum located between the two pulleys, and the pulleys are swung about their fulcrums to change the speed ratio between the input and the output shaft. A band tensioning shaft, extending parallel to the input shaft and output shaft and between the shafts is frequently provided. Fulcrum blocks, or members, are threadedly engaged with the shaft (which has threads of opposite hands at opposite ends of the shaft) to define the fulcrums for the levers. Rotation of the shaft in one direction will draw the sides of both pulleys together to simultaneously increase their diameters, and thus tighten, or increase the tension in, the band. Transmissions of this type are shown in U.S. Pat. Nos. 2,112,157; 2,266,687; 3,097,540; 3,136,169; 3,138,033; 3,190,136; 3,349,633; 3,490,301; 3,750,487; and 3,916,705.
All of the above noted patents have automatic band tightening, or tensioning, mechanisms to increase the diameters of both pulleys simultaneously as wear occurs in the band, thereby maintaining the desired tension in the band. Sometimes the automatic tightening is effected by a coil spring engaged with the band tensioning shaft as shown in U.S. Pat. Nos. 2,112,157; 3,136,169; 3,750,487; and 3,916,705.
If a coil spring has one end connected to the band tensioning shaft to bias the shaft in a tightening direction, it is necessary to anchor the opposite end of the coil spring. At the same time, it is desirable to release the other end of the coil spring for adjustment and for resetting the mechanism when a new band is installed. The mechanism of U.S. Pat. No. 3,916,705 includes a bevel gear wheel rotatably mounted on the shaft, and a winding gear engaged with said bevel gear wheel. The shaft on which the winding gear is mounted extends outside the transmission housing and can alternatively be manually rotated or locked in a fixed position.